IRIDESCENT POLYCHROMATISM
IN A FEMALE
HUMMINGBIRD:
IS IT RELATED TO
FEEDING STRATEGIES?
ROBERT BLEIWEISS l
Museum
ofComparative
Zoology,
HarvardUniversity,
Cambridge,
Massachusetts
02138USA
ABSTP,
ACT.--Conspicuous
coloration
mayevolvethroughaggressive
competition
eitherfor
mates,in sexualselection,or for any limiting resource,in socialcompetition.Most species
of hummingbirds
(Trochilidae)
havepolygynous
(promiscuous)
matingsystems,
brightmale
plumage,andconspicuous
sexualdichromatism.
Thesepatternshaveled to the view that
their iridescentplumagehasevolvedby sexualselection.
I reportan iridescentpolychromatismin femalesof the AndeanhummingbirdHeliangelus
exortisexortis
that suggests
the
evolutionof iridescentplumagevia aggressive
feedingbehavior.FemaleH. e. exortis
vary
within and amongpopulations
in the development
of the largeiridescentgorgetcharacteristicof males.I documenta regularnorth-south
patternof geographic
variationin the polychromatism.In samplesfrom northernColombia,femalegorgetsvary from a noniridescent
(femalelike)to a fully iridescent(malelike)condition,whereasfemalesfrom southernColombiahaveonlythediscrete
noniridescent
andfully iridescent
morphs.Samples
of females
from Ecuadorare comprisedpredominantlyof the noniridescent
morph.The colorforms
are not due to seasonal
plumagedifferences
or ontogenetic
changes.A significantnegative
correlation
betweenbill lengthanddegreeof iridescence
in the largestgeographic
sample
suggests
thatfemaleswith iridescent
gorgetsare moreaggressive
territorialforagers
than
lessiridescentfemales.Received
16 July1984,accepted
8 April 1985.
NUMEROUS
factorshave been proposedto ex- socialcompetition(Crook1972;West-Eberhard
plain the evolution of conspicuouscoloration 1979, 1984) recognizes that analogousinteracin animals.Theseinclude predatorwarning and tionsmay extendto competitionover limiting
avoidance (Curio 1976, Baker and Parker 1979, resourcesother than mates, and promote the
signalsusedin nonEndler 1979), thermoregulation (Watt 1968, elaborationof conspicuous
Clark 1974, Walsberget al. 1978), and behav- sexual contexts.
The effects of sexual selection and of nonsexioral communicationas influencedby the biotual social competition are difficult to distinic (Nobel and Vogt 1935,Tinbergen 1951,Murbothphenomenalead
ray 1981) or physical (Burtt and Gatz 1982) guishin practicebecause
environment. Evaluationof the relative impor- to the predictionthat increaseddominanceand
tance of these potential influences presentsa aggressivebehaviorscan lead to the elaboration of conspicuous
signalssuchasbright colmajor challenge to evolutionists.
Within the category of behavioral commu- oration. Moreover, the effects of sexual selecnication, the effects of nonsexual social com-
tion are difficult
petition are particularly difficult to distinguish
reproducinganimals.Specifically,variation in
male coloration(e.g. Rohwer and Niles 1979,
Dominey 1980,Ewald and Rohwer 1980,Gross
from those of sexual selection. Darwin (1871)
proposedthat conspicuous
sexuallydimorphic
featuresevolve via sexuallyselectedcompeti-
to control
in sexually
and Charnov 1980, Rohwer et al. 1980, Procter-
tion for mates, through the advantage these
featuresconfer either in mate choice (epigamic
selection) or in aggressiveinteractions among
Grey and Holmes 1981, Edwards1982, Flood
members
access to
invariably are tied to breeding activities(Wil-
the other (intrasexual selection).The theory of
son 1975, O'Donald 1977). This ubiquitous cor-
of one sex that determine
1984) often fails to distinguish these hypotheses becausemale dominance and territoriality
relation
makes it difficult
to determine
wheth-
er nonsexualsocial competition is a sufficient
• Present address:Department of Ornithology,
American Museum of Natural History, New York,
New
explanationfor the evolutionof conspicuous
coloration
in males.
The occurrence of intraspecific variation in
York 10024 USA.
701
The Auk 102: 701-713.
October
1985
702
ROBERT
BLEIWEISS
female colorationprovidesa natural control of
this difficulty that has gone unappreciated.In
many species,especiallythose in which males
mate promiscuously,females are less aggressive sexually,thereby providing somecontrol
for the influence
of sexual selection.
Variation
in female plumage is particularly relevant for
determining the evolutionary significanceof
iridescentplumagein the largely promiscuous
hummingbirds (Trochilidae), which exhibit
someanomalousfemale plumagesthat are difficult to explain by sexualselection.
Male hummingbirdsemploy their iridescent
plumage in a variety of contexts,including
nuptial displays,aggressivesexualdisplays,and
aggressivedisplaysassociatedwith nectar-centered feeding territoriality (Stiles 1973, 1982).
Because promiscuous breeding systems are
thought to be associatedwith high levels of
sexualselection(Payne 1984),the bright male
plumageand sexualdichromatismcharacteristic of most hummingbirds are consistentwith
sexualselectiontheory (Selander 1972). However, sexualselectiondoesnot explain the evolution of "bright monomorphic" speciescharacterizedby a brilliant female plumage similar
to that of the male (e.g. in the generaPanterpe,
EuIampis,
AmaziIia).Bright monomorphicspecies
do not show
a consistent
reduction
in the in-
tensityof sexualselectionasinferred from their
breeding systems(Wolf and Stiles 1970, Wolf
1975a).Rather, both sexesin bright monomorphic speciesexhibit nonbreeding nectar-centered feeding territoriality (Wolf 1969, 1975a,
b; Ingles 1976).Feedingterritoriality is usually
lacking in the dull-coloredfemalesof mostsexually dichromaticspecies(Wolf 1969, 1975a,b;
Linhart 1973). Therefore, the correlation be-
tween bright female plumageand feeding territoriality strongly suggeststhat conspicuous
iridescencemay evolve solely for its importance as a nonsexualaggressivesignal (Pitelka
1942, Wolf and Stiles 1970, Stiles 1973).
In this paper I addressa previously unanalyzed color pattern that involves the variable
expressionof iridescentmalelike featuresin females of an Andean hummingbird, the Tourmaline Sunangel (HeIiangelusexortis exortis).
Subtle examplesof this "polychromatic"variation in hummingbirdshave been noted in the
temperate species Calypte anna (Williamson
1956)and in severaltropical genera,including
HeIiangelus(Berlioz 1944, 1949), but distinctive
femalevariantsin iridescentmale plumageare
[Auk,Vol.102
recordedonly for the tropicalWhite-neckedJacobin (FlorisugameIIivora;Zimmer 1950b, Mon-
roe 1968, Elgar 1978).Although H. e. exortisis
consideredto be sexuallydichromatic,with a
highly iridescentgulat patch or "gorget" in
malesand a white gorgetin females(e.g.Meyer de Schauensee1964),Chapman(1917)noted
that some females of H. e. exortis from the West-
ern Cordilleraof Colombiahad fully iridescent
gorgetslike those of males. I show here that
femaleH. e.exortis
vary dramaticallybothwithin and among populationsin the malelike iridescence
on their gorgets,and I arguethat the
polychromaticvariationsupportsthe origin of
iridescentcolorationin hummingbirdsthrough
selectionfor aggressivesignalsassociated
with
foragingbehavior.
MATERIALS AND METHODS
In Hellangelus
exortis,the violet-chinned and rosethroatednominateform, H. e. exortis,
occursthroughout the three Andean
Cordilleras
of Colombia
and
through the EcuadorJanAndes, primarily on the
eastern slope. Two orange-throated forms, H. e. micraster and H. e. cutervensis,occur in the Andes of
southwesternEcuador and northern Peru, respectively (Peters 1945, Zimmer 1951), but are rare in
collections
and are not treated
here. I examined
275
individuals from the entire range of H. e. exortisin
this study. Sample size and geographicdistribution
of specimensare given in Table 1. A more complete
descriptionof the geographicrelationshipsamong
localities can be found in Bleiweiss(1985).
Determinationof ageand sex.--Only two specimens,
both immatures,had gonad data noted on their labels. Therefore, alternative methods had to be used
to determineage and sex.To distinguishimmatures
from adults,I relied on the presenceof corrugations
on the culmensof immaturebirds (Ortiz-Crespo1972,
Stilesand Wolf 1974).Specimenswere examined under a Bauschand Lomb zoom dissectingmicroscope
(7-30 x) to look for the presenceof corrugations.A
specimen was considered adult if the bill was perfectly smooth outside the nasal depression.The inverse relationship between the number of bill corrugationsand age of immature birds permits one to
study plumage ontogeny (Ortiz-Crespo 1972, Ewald
and Rohwer 1980).To determinethe relative age of
immatures,bill corrugationswere counted with the
aid of a Wild microscopewith cameralucida attachment. Wing and tail measurementsare considerably
larger in males of many except the smallest hummingbird species(Zimmer 1950a, 1952; Stiles 1983;
Payne 1984). I have verified this relationship for Heliangelus
with largeseriesof H. amethysticollis
for which
gonadal data were available (Bleiweissin prep.). To
determine the sex of H. e. exortisspecimens,I em-
October 1985]
703
FemaleVariationin Iridescence
TABLE1. Distribution and samplesize for Heliangelus
exortisexortisspecimensexamined.
Cordillera
Locality
Pooled'
West
Central
b
East
Adults
1. El Pefi6n
A
West
2. La Aguadita
A
West
1
3. E1 Robie
A
West
6
4. Fusugasug•-Silvania
A
West
4
5. Hacidenda
1
2
6
17
1
10. Laguneta
West
43
15
11. Toche; Tolima
12. Purac•
West
West
West
West
7
1
6
West
6
B
B
13. Coconuco
14. Paletara
15. Almaguer
16. Hacienda
La Ilusi6n
17. La Florida
18.
19.
20.
21.
Coast Range
Cerro Munchique
Munchique-E1Tambo
Tijeras-Moscopin
7
1
East
East
West
West
West
6.
7.
8.
9.
Zuliaba
Santa Elena
P•ramo de Sons6n
E1 Zancudo
Santa Isabel
ttlres
1
2
1
1
1
2
C
West
East
8
1
C
C
C
East
East
East
3
25
2
7
4
East
1
1
22. Guanderal
23. Cerro Pax
D
D
East
East
3
8
5
24. Papallacta
25. Cuyujfia
E
E
East
East
8
14
4
26. Baeza
27. Volcln
E
East
Isolated
3
10
1
1
28. San Antonio
29. Ambato
30. Barios
F
F
F
East
East
East
1
1
6
1
31. Volcln Tungurahua
F
East
4
32. Planchas
33. Coraz6n
F
East
West
Sumaco
peak
34. Pallatanga
35. Macas
West
F
East
Total
1
2
1
6
3
4
219
56
Letters A-F designatepooled localities used for analysisof geographicvariation (see text).
East and west designationsunder each Cordillera refer to the slope of the range.
ployed the two measurementsthat best separatedthe
sexesin H. amethysticollis:
length of unflattenedouter
primary from point of emergencefrom skin, and
length of outer tail feather (rectrix 5) from point of
emergencefrom skin. The extrapolationfrom H. ame-
thysticollis
to H. e. exortisis justifiedbecausethey are
congeners. There is no reason to assume that what
holds for H. amethysticollis
and many other hummingbirds doesnot also hold for H. exortis.
Morphological
variation.--I use the term "gorget" to
denote the entire gular region, and "chin" and
"throat" to denoteits uppermostand remainingportions, respectively.The color variation describedinvolvesiridescentstructuralcolorsthat vary in intensity: "glittering" indicates intense, mirrorlike
iridescence,while "shining" indicateslessintenseiridescence with a metallic luster (Greenewalt 1960).
The violet chin and rose throat feathersare always
glittering in both sexesof H. e. exortis.Nonglittering
chin and throat feathersvary geographicallyfrom
white (eastern and southern Colombia through Ecuador) to white with a shining green to dusky terminal dot (northern and central Colombia) [see Bleiweiss (1985) for a discussionof this variation]. The
female polychromatism involves variation in the
number of glittering gorgetfeathers.I quantified the
polychromatic variation either by directly counting
the number of glittering iridescentfeathersor, when
very numerous, by estimating their numbers by
countingthe tracingsof all glittering gorget feathers
made with
the camera lucida.
To quantify the pattern of geographicvariation in
glittering gorget iridescence,I divided the variation
in glittering feather counts into four gorget color
704
ROBERT
BLE•WE•SS
>
Fig. 1. Distribution of glittering gorget feather
countsfor all adult females.Qualitativegorgetcolor
classesII-IV were designatedon the basisof the two
largestdistributionalgapsin feathercounts(between
18 and 20 and between29 and 32, as indicatedby
dashed lines).
[Auk,Vol.102
tionshipbetweenpolychromaticvariation and feeding behavior:(1) length of exposedculmen (EC), (2)
length of culmenfrom flangeof the nasaloperculum
to tip (CF), (3) length of outerprimary (FP), (4) chord
of unflattened wing from the bend to the tip (WB),
(5) length of inner (IT) tail feather (rectrix 1), and (6)
length of outer (OT) tail feather(rectrix5) (seeBaldwin et al. 1931). I used the two measuresof wing
lengthasindicesof wing discloading(assumingequal
weights among females)becauseweights were not
available.Specimens
were measuredunderthe Bausch
and Lomb zoom dissectingmicroscopewith needlepoint dividers, that were then remeasuredwith dial
calipers. I omitted individual measurementswhen
featherswere molting or very worn.
Statisticalanalysis.--Because
of small sample sizes,
specimensfrom severallocalitieswere pooledfor statistical analysisof geographicvariation (pooled localitiesare designatedby lettersA-F in Table1). Close
geographicproximity and absenceof obviousphysical barriers
between
localities
were the criteria
used
for pooling(Bleiweiss1985).Geographicvariationin
gorgetcolor classeswas analyzedby R x C contingency tablesusing Chi-squarestatisticswith at least
one expected observation per cell (Shedecor and
Cochran 1969:235, 241). I calculatedSpearmanrank
classes(I-IV) based on the number of glittering
feathers.ClassI denotesabsenceof glittering iridescence.The frequencydistributionof glittering feather countsamongall adult females(n = 94) revealsan
uneven distribution, with gapsat 18-20, 29-31, and
correlation coefficients between mensural characters
46-49 (Fig. 1). The first two gaps help divide the
and
gorget coloration for each geographic sample
sampleinto three qualitative categories(little, modseparately
becausethere is significant geographic
erate, and full iridescence)and permit the assignment of individuals to different count classes(class variation in most of the measurements (Bleiweiss
II = 1-17, class III = 21-28, class IV = 32+). These
classesare consistentwith a subjectiveimpressionof
differencesin appearance.
Classes
II and III haveboth
glitteringandnonglitteringfeathers.ClassI hasonly
nonglitteringfeathers,and classIV usuallyonly glittering ones.
1985).
RESULTS
Sexual
variation
Mensural charactersmay be correlatedwith differences in feeding behavior between the sexesand iridescence.
among different hummingbird species(Feinsinger
and Colwell 1978). I used such characters as an in-
directtestof the relationshipbetweenpolychromatic
variationand feedingbehaviorin the absenceof behavioral observations.Territorial speciesin general,
and malesin particular,often have shorterbills, and
shorterwings relative to body mass(wing disc loading), than nonterritorial speciesand females,which
are lessaggressive(Stiles1973,Feinsingerand Chaplin 1975, Feinsinger and Colwell 1978, Snow and
Snow 1980). These differencesin morphology are
thought to increasethe energeticefficiencyof territorial and nonterritorial foraging strategies(Wolf et
al. 1972,Feinsingerand Chaplin 1975,Feinsingerand
Colwell 1978, Ewald and Williams 1982). Similar
morphologicaldifferencestherefore may evolve together with evolutionary divergencein foraging behavior among individuals.
I used six charactersto examinethe possiblerela-
in adult
Colombian
and Ecua-
doriansamples.--Adult Colombian H. e. exortis
show continuousvariation in glittering gorget
Adult
H. e. exortis from Ecuador
ex-
hibit only two gorget conditions:glittering violet chin and rose throat, or pure white chin
and throat.
Because variation
in Ecuador
is sim-
plified, I used these samplesas the standardfor
determining sex in the variable Colombian
samples.A bivariate scatterplot of wing length
(FP) vs. tail length (OT) for all adult specimens
from Ecuador(localitiesE, F, 27, 34) separated
adults into two groups (Fig. 2A). Individuals
with wing and tail measurementsgreater than
45.5 mm have highly iridescent gorgets,which
are violet
on the chin
and rose on the throat.
Smaller individuals have pure white, noniridescent
chins and throats
in 24 of 25 individ-
uals. This clear morphological division in the
EcuadorJansamplereflectssexualdifferencesin
measurements.Thus, all males have highly ir-
October1985]
FemaleVariationin Iridescence
705
39-
35
37
39
4l
43
45
47
49
5'l
5'3
35
LENGTH
Of OUTERTAILFEATHER
(ram)
37
39
4l
43
45
47
49
5l
LENGTHOFOUTER
TAILFEATHER
(ram)
41'
LENGTH
OFOUTER
TAILFEATHER
{ram)
LENGTH
OFOUTER
TAIL
FEATHER
{ram)
Fig. 2. Scatterplotsof lengthsof outertail feather(OT) vs. first (= outer)primary(FP) for Ecuadorian
localities
E, F, 27, 34 (A), and the threelargestColombian
samples:
B= loc B (El Zancudoregion),C =
Laguneta,D = loc C (Munchiqueregion).Solidsymbolsare males;opensymbolsare females.The number
of glitteringgorgetfeathers
is indicated
aboveeachcircle.Triangles
arefemales
withno glitteringgorget
feathers.
Specimens
for whichonlyoneof the measurements
couldbe takenare alsoplotted;theyfall on
theappropriate
axis.Arrowsindicatequestionable
females(seetext).Scatterplotsof additionallocalities
can
be found in Bleiweiss(1983).
idescentgorgetsand all femalesbut one have
pure white gorgets.
In addition to the gorget color differences,
the sexesalsodiffer in body colorationin Ec-
body coloration.Plotsof Colombianspecimens
for wing length (FP) and tail length (OT) fall,
like EcuadorJan
ones,into two groupsthat are
roughly separatedby a size of 45.5 mm for FP
uador.Althoughbothsexesarepredominantly and OT (Fig. 2B-D). Largerindividuals(males)
shining green,malesare much darker than fe- have the samegorget color as malesfrom Ecmales,with a bronzecaston the backand up- uador. The female portion of each sample,
per tail coverts. Male crowns look blackish however,is highly variable,with from 0 to 55
when seenfrom the front and havea highly glittering iridescentgorget feathers.For indicontrastingglittering green forecrown. Female vidualssexedby bodyplumage,2 of 75 females
crownsare green,with lesscontrastingglitter- (indicatedby arrowsin Fig. 2C, D) and 7 of 104
ing forecrowns. Female belly feathers are maleshadoneof the two measurements
larger
broadly edged with tan, and their outer tail or smaller than expectedbasedon their plumfeathershavelight tips, featuresthat are absent age.Of these,the sexof 4 of the malesand the
in males.
classI femalefrom Laguneta(Fig. 2C) areprobColombianspecimenswere first sexedby ably correctbecausemy evaluationagreeswith
706
ROBERT
BLEIWEISS
75'
78'
81'
[Auk,Vol.102
72'
T
r2ø
WESTERN
CENTRAL
i0 ß
CLASS
8o
sß
œo
'"'"'•o
N=,O
ß"
IOOO
METER
CONTOU
•
/ ;
•
78 ø
75 ø
OVER
3000METERS
, I00 KILOMETERS
4o
72 ø
Fig. 3. Geographicvariation in glittering gorget iridescencein adult female H. e. exortis.Gorget color
classesare basedon Fig. 1 and are as describedin text. Separatehistogramsfor eachlocality are on the left
side.The principalgeographicpatternis summarizedby the histogramson the right side.The Bogot•sample
(top center)has all four gorget color classesand is usedas a key for the other histograms.Localitieswere
plottedwith the aid of ornithologicalgazetteers(Paynterand Traylor 1977,1981).Numbersunderhistograms
refer to samplesincluded (seeTable 1). There was no significantdifferentiationamonglocalitiesin the central
sectionsof the Central and Easterncordilleras(A, B, and loc 10;3 x 4 contingencyX2 = 0.809,v = 6, NS).
the sex noted on labelsby the collectors,who
sexedbirds by examiningtheir gonads(Chapman 1917: 304-305). The other three problematicalmalesare all fully iridescentand do not
affect the general conclusions.
Femaleswith the highestcountsof glittering
gorget feathershave gorgetsthat are malelike
in color, but this color is more limited in extent
having broad white or light gray concealed
bases,and in having violet chin feathersonly
when the glittering gorgetiridescenceis extensive.Although the numberof glittering gorget
feathers in males also varies, this variation is
not related to qualitative changesin gorget color and pattern. Within the accuracyof the sex-
ing criterion,adult malesfrom all geographic
regionshave the gorget color and pattern describedfor malesfrom the Ecuadoriansample.
detailfrom thoseof typicalmalesin frequently The variation in number of glittering gorget
(maximum count 56 vs. a range of 43-95 for
males). Iridescent gorgetsin females differ in
October1985]
FemaleVariationin Iridescence
707
feathersfor combinedadult femalesamples
was
significantly greater than that for males [F-test
of In (numberof glittering feathers+ 1) (Sokal
and Rohlf 1981: 421), F = 14.88, v• = 93, v2=
113;P < 0.001 (one-tailed test)].
Geographic
variationin femalecoloration.--The
pattern of geographic variation in all four
classes
of glittering iridescenceamongadult femalesis summarizedin Fig. 3. There is striking
geographicdifferentiationin the best-sampled
region from the Central Cordillera of Colombia
southward into Ecuador.Samplesare highly
variable (classesI-IV) in the north, but exhibit
decreased
variabilitysouthward,with only the
extreme noniridescent and fully iridescent
morphspresentin southernColombiaand with
NUMBER OF BILL CORRUGAT)ONS
NUMBER Of BILL CORRUGATIONS
Fig. 4. Ontogeneticvariation in glittering gorget
iridescenceamong immaturemale (left) and female
(right) H. e. exortisfrom combinedgeographicsampies.
decreasingfrequencyof the iridescentmorph
(classIV) in Ecuador.Differencesamong sampieswithin this region were highly significant
(3 x 4 contingency table, X2 = 48.265, v = 6,
P < 0.001).The occurrenceof only the two ex-
bill corrugations disappear. Males with <20
corrugationsalready have gorgets similar to
those in smooth-billedmales (immature range
36-76, adult range 43-95). Immature females
treme gorget classesI and IV in the southern with comparablenumbers of bill corrugations
section of the Central Cordillera of Colombia,
(<20) exhibit the full range of variation obthough based upon small individual collec- servedamong smooth-billedfemales.Taken as
tions,is repeatedfor the severalcontiguouslo- a whole, this information doesnot rule out agecalities. Other small collections from elsewhere
relatedor other facultativechangesin adult female gorgetcoloration,but it doesindicate that
in the range do not show this pattern.
Two other relatively well-sampled regions any gorget coloration may exist at the incepalsoexhibit suggestivepatterns.The sampleof tion of maturity.
I found no evidence of significant seasonal
femaleH. e. exortiswith accuratelocality information from the Bogot• region was limited
or annual changesin gorget color classfre(n = 7), but the high variability of femalesfrom quencies.Within localities,the range of polythis region (classesI-IV) is further supported chromaticvariantsin subsamples
collectedover
by their high variability among the numerous a brief time interval is similar to that for the
Bogot• trade skins. Trade skins were not inentire sampleand for other large subsamples
cluded in the analysisbecausethey lack spe- collected during different months and years
cific locality information. These collections (Table 2). For example,mostof the large, varigenerally were made in the Bogot• region or, able Laguneta sample (locality 10) comprises
lessfrequently, in the centralpart of the Cen- two collections obtained in different months
tral Cordillera (Chapman 1917, Berlioz and and years,onefrom Augustto September1911,
Jouanin 1944). Bogot• trade skins also exhibit the other from March to April 1942.There was
all four gorget classes.Unlike all other large no significantdifference(by Chi-square)in the
samplesof females,the samplesfrom the Cerro frequencyof the different gorget color classes
Munchique region (loc C) lack individuals in betweenthesetwo samples.Similarly, the prethe noniridescentclass(I).
dominantly white-gorgetedsamplesfrom EcAge and temporalvariation in gorget irides- uador have been collected in various months
cence.--Theontogenyof plumagecolorationin and years (see Table 2), indicating that the
H. e. exortisindicatesthat the adult plumage, prevalence of classI is not due to temporal
and the completerange of variability in gorget samplingbias.This particularpattern was stacolorationamong smooth-billedfemales,is at- ble over at least 41 years (seeTable 2). These
tained before the bill corrugationsdisappear dataalsoindicatethat the basicpattern of geo(Fig. 4). Adult maleshave a single gorget color graphicvariation is not the result of temporal
that iramaturesacquire gradually before their sampling bias (Table 2).
708
ROBERT
BLEIWEISS
[Auk, Vol. 102
TABLE
2. Temporalsamplingof adult polychromaticvariantswith respectto month and year.a
Total
classes
Locality
n
A. Bogotfi
region
7
Month
Year
1913
I-IV
I-IV
(2, i, 2, 2)
7
2-31 Aug
1918
I-IV
I-IV
(2, 2, 1, 2)•
10. Laguneta
9
4 Aug-6 Sept
1911
I-IV
I-IV
(5, 1, 2, 1)c
15. Almaguer
9
2
17 Mar-15 Apr
11-16 Mar
1942
1912
I-IV
I, IV
I-IV
I, IV
(1, 1, 5, 2)
(1, 0, 0, 1)
C. Munchiqueregion
21. Tijeras-Moscopfin
4
2
15-22Apr
23 Feb-3Mar
1957
1952
II-IV
I, IV
II-IV
I, IV
(0, 2, 1, 1)
(1, 0, 0, 1)
23. Cerro Pax
4
20-23 Dec
1950
I, IV
I, IV
(2, 0, 0, 2)
24. Papallacta
25. Cuyujfia
2
2
Feb
Aug
1899
1932
I
I, IV
I
I, IV
(2, 0, 0, 0)
(1, 0, 0, 1)
26. Baeza
27. Volcin Sumaco
5
3
2
19 June
2-10 Nov
12-14 June
1928
1922
1924
I, IV
I
I
I
I
I
(5, 0, 0, 0)
(3, 0, 0, 0)
(2, 0, 0, 0)
31. VolcanTungurahua
34. Pallatanga
2
2
June
19-20 Oct
1939
1931
I
I
I
I
(2, 0, 0, 0)
(2, 0, 0, 0)
8. E1Zancudo
24 Mar-6 Apr
for locality Total classesfor sample
' Localitylettersand numbersrefer to designations
in Table 1. Numbersin parentheses
are the numberof
individuals in each of the four gorget color classes(I-IV).
• Locality8 vs. locality 10, Aug; Yatescorrectedfor continuityx2 = 0.29, v = 3, NS at 0.05 level.
cLocality 10, Aug vs. locality 10, Mar-Apr; Yatescorrectedfor continuity x2 = 2.57, v = 3, NS at 0.05 level.
Trophicmorphology.--Inthe largestpolychromatic female sample,from Laguneta (locality
10, n = 21; all other polychromatic localities,
n < 10),bills vary by morethan 2 mm in length
(EC 15.3-17.6),and both measuresof bill length
were significantly negatively correlatedwith
numbersof glitteringfeathers(Table3). There
were no significantcorrelationsbetween gorget color and the other four measurementsat
Lagunetaor in any characterfrom other large
samples(n > 4). Male and female classIV individuals from Lagunetawere also similar in
bill length (for example,for EC, Mann-Whitney U-test;U, = 52, t, = 0.77,v = 23; two-tailed
unlikely explanation for the origin of polychromaticvariation in female hummingbirds.
By contrast,aggressivecompetitionfor nectar
mayoccurin both sexesin hummingbirds,and
it helps explain the evolution of the brilliant
femaleplumageof bright monomorphicspecies
(Wolf 1969, Wolf and Stiles 1970). These considerationssuggestthat competition for nectar
is a more plausible basisfor the origin of po-
lychromaticvariationin femaleH. e.exortis
than
is sexual selection.Social competition is de-
test, P > 0.5), while bills of classI females were
fined in terms of intraspecific competition,
whereas aggressivecompetition for nectar in
hummingbirdsmay include interspecificinteractions(Moynihan 1979). I therefore refer to
significantlylonger than bills of males(U, =
the hypothesisfor the evolution of polychro-
matic variation via nonsexualcompetition for
Thus,in the Lagunetasample,individualswith nectaras the "feedingstrategyhypothesis"in
iridescentgorgetsin either sexhave compara- order to include the possibleeffectsof interble bill lengthsthat are shorterthan in females specificaggression.
The principal prediction of the feeding stratwith nonglitteringgorgets(classI).
egy hypothesisis that polychromaticvariation
in the gorgetof femaleH. e. exortisreflectsvariDISCUSSION
ation in foragingbehavior.Iridescentfemales
In promiscuous mating systems,which are of bright monomorphichummingbirdsactivecharacteristic of most hummingbird species ly defendtheir territoriesand are codominant
94, t, = 2.69, v = 24; one-tailed test, P < 0.01).
(Stiles and Wolf 1979), the potential for sexual
selectionas measuredby variancein reproductive success
is high in malesbut low in females
(Payne 1984). This makes sexual selectionan
with males (Wolf 1969, Stiles and Wolf 1970,
Wolf and Stiles 1970). On this basis, I further
predict that increasing gorget iridescence
should be positively correlatedwith increasing
October1985]
FemaleVariationin Iridescence
709
T^I•LE3. Spearmanrank correlationcoefficientsfor female gorgetiridescenceand six mensuralcharacters
for the Lagunetasample(locality 10).a Sample sizesare given in parentheses.
EC
CF
FP
WB
IT
OT
-0.59** (21)
-0.51' (21)
0.32 (20)
0.37 (17)
0.29 (20)
0.21 (19)
' EC = length of exposedculmen, CF = length of culmen from flange of nasal operculumto tip, FP =
length of outerprimary, WB = chordof unflattenedwing from bend to tip, IT = length of inner tail feather,
OT = length of outer tail feather. * P < 0.02, ** P < 0.01 (two-tailed tests).
dominanceand degree of feeding territoriality
among female H. e. exortis.There is as yet no
direct proof of these predictionsfrom field observations.However, aggressiveterritorial exploitation of nectar is characteristicof male H.
e. exortis(Moynihan 1979, Hilty and Brown in
press).In other territorial hummingbird species
where males have a glittering gorget, the gorget is used in aggressivedisplays associated
with feeding territoriality (Ewald and Carpenter 1978). Feeding territoriality in male H. e.
exortistherefore provides a necessarycondition
for my inference that female H. e. exortiswith
iridescentgorgetsare territorial nectarfeeders.
The negativecorrelationbetweenbill length
and the degreeof gorget iridescencein females
from Lagunetasupportsthe predictedassociation of an iridescent gorget with aggressive
feeding territoriality (in light of the possible
general relationship between decreasingbill
length and increasedterritoriality discussedin
variation. The polychromatic variation describedin this paper differs from ontogenetically developedcolorvariation in at leastthree
importantrespects:(1) occurrencewithin broad
age categoriesof immatures and adults, (2)
temporalstabilityin the relative frequenciesof
the gorgetcolor classesat somelocalitiessuch
as Laguneta,and (3) female sex-biasedpattern
(polychromaticvariation limited to females).I
explorebelow the possibleimplicationsof these
characteristics
for the feeding strategyhypothesisto provide a specificframework for future
research.
Age independence.--Ageindependence of
polychromaticvariantscontrastswith the strong
ontogeneticcomponentof many casesof variable male plumage ascribedto sexual selection
(Gilliard
mented
1969, Wilson 1975). Dull, unornamales are often subordinate
that do not achieve
full
immatures
ornamentation
or re-
productive potential until they move up the
the Materials and Methods). However, this cor- dominance hierarchy. Such males may even
relation is not consistentamong all localities. delay attemptsto reproduceuntil they are oldPerhaps,variation in bill length is not neces- er, but may gain in lifetime reproductive fitsary for the evolution of individual differences nessby achieving greater reproductivesuccess
in the degree of feeding territoriality.
at an older age. Thus, sexually selectedplumThe only direct observationsof behavioral age variation in males may arise out of its recorrelatesof intrapopulationalvariation in gor- flection of age rather than from the selective
get coloration involve ontogenetic variation in value of different levels of ornamentation per
male Calypteanna.Adult male C. annahave glit- se (Price 1984). The appearanceof polychrotering red crowns and gorgets that develop matic variation early in the ontogeny of H. e.
gradually with age. As is consistent with the exortis,and its persistencein adults, suggests
predictionsfor polychromaticvariation in H. e. that the different phenotypesare viable at any
exortis,adult male C. annaare territorial during age. Competition for nectar may produce agethe nonbreeding seasonwhile the less irides- independent polychromatic variation in fecent juveniles hold poor-quality territories or males; unlike competition for mates among
are not territorial (Ewald and Rohwer 1980). It
males,femaleshave no apparent fitnesspayoff
is difficult, however, to assessthe evolutionary for delaying accessto nectar.
significanceof polychromatismsusing ontogeTemporalstability.--Temporal stability of
netic variation becauseindividuals differ in age polychromatic variants suggeststhat they can
(experience)and sexual maturity. These two coexistwithout evolutionary exclusion.Coexisfactors can determine the outcome of aggres- tence of polychromaticvariants that differ in
sive interactions
even in the absence of color
foragingbehavior is possibleif bright plumage
710
confers
ROBERT
BLEIWEISS
different
costs and benefits
from
dull
plumagein different socialcontextsassociated
with foraging (Ewald and Robwet 1980, Robwet 1982). For example, bright plumage, although possibly advantageousin aggressive
encounterswith conspecificsor other species,
may be costly for subordinate birds if it increasesthe rate and intensity of attacks by
dominants. Conversely, dull plumage in subordinatesmay increasetheir successat intruding onto nectar territories and reduce the severity of aggressiveresponsesby territorial
birds (Ewald and Robwet 1980). Further morphologicalevolution for increasedefficiencyin
different foraging strategiesby dominantsand
subordinates
should
enhance
stable
coexis-
tence. This possibility is suggestedby the correlation between bills and coloration in Laguneta, where the frequency characteristicsof the
gorget color classesdid not changesignificantly over at least 30 years. Temporal and geographic shifts in polychromaticvariation may
be causedby extrinsicshifts in nectar resource
availability through changes in the flora or
changes in avian nectar competitors such as
other hummingbirds or the nectarivorousDiglossa(Coeribidae).
Femalesex-biased
pattern.--Intersexual differencesin variability may be due to sexualselection. The sex bias in the polychromatismrepresentsa simplecaseof reducedmale variability
documentedin many taxa (Pearseand Murray
1982,Stampsand Gon 1983).Stabilizingsexual
selection by female choice and intrasexual
aggressionhave been proposedas alternative
explanationsof this pattern (Pearseand Murray 1982,Robwet 1982,Stampsand Gon 1983).
Both processesmay operate in conjunctionto
reducevariation in aggressivesignalsin male
hummingbirds.Femalehummingbirdsof some
speciesseem to choosemales on the basis of
occupancyand quality of a nectarterritory (e.g.
Calypteanna,Panterpeinsignis;
Stiles1973).This
indirect female choiceof male aggressioncan
lead to strong selectionfor aggressiveness
and
aggressivesignalsamongall malesif all males
compete for territories during the breeding
seasonwithout adopting alternative nonterritorial mating strategies(Robwet 1982).Indeed,
these conditions appear to exist in Calypteanna
(Stiles 1973), where most males are iridescent,
[Auk, Vol. 102
appearto be lessdiscriminatingin mate choice
(Sibley 1957). Thus, male choiceacting on femaleswould be lesslikely to eliminate female
color morphs that evolved through nonsexual
selection.
Both proximate mechanisms and broader
ecologicalcausesprobably will be important
for analyzing the evolution of hummingbird
polychromatisms.Hummingbirds as a whole
meet the requirementsfor the evolution of intrapopulationalvariation in conspicuouscoloration as outlined by Robwet (1982):presence
of aggressivecompetitionfor a resource(nectar), variation in dominance,and frequent aggressiveinteractionsamong individuals. The
simplestmechanismfor the origin of variation
in phenotypic traits associatedwith variation
in dominancebehavior is a physiologicallink
between coloration and behavior (Keeler et al.
1970, Watt et al. 1984). The final form of the
variation may then depend upon how alternative individual strategiesof resourceexploitation are limited by resource availability.
Presenceand absenceof territoriality in hummingbirdsdependsupon nectarresourcedensity, with territoriality around denser clumps
of nectar. Therefore, female polychromatisms
may evolve when femaleswithin a population
forage over a broad range of nectar densities,
while sexually dichromatic and bright monochromatic speciesmay evolve when respective
femalesspecializeat low or high densities.
ACKNOWLEDGMENTS
I thank my thesisadvisor,E. E. Williams, for many
helpful discussionson general evolutionary topics
over the years and W. Bock,W. Bosseft,P. Ewald, P.
Feinsinger,S. Rohwer, J. Shopland, F. Vuilleumier,
and L. L. Wolf for helpful criticism on earlier drafts
of this paper. I gratefully acknowledgethe helpful
discussionand commentsof K. Fristrup, D. Geary, A.
Haynes,and G. Mayer. I thank the following curators
for kindly providing specimensunder their care:L.
Baptista(California Academy of Sciences),F. Gill
(Academy of Natural Sciences,Philadelphia), J. C.
Hafner (Moore collection, Occidental College), J.
Jennings(WesternFoundationof VertebrateZoology), R. A. Paynter (Museum of Comparative Zoology), J. V. Reinsen (Louisiana State University Museumof Zoology),R. Schreiber(LosAngelesCounty
Museum), L. L. Short (American Museum of Natural
History), S. Wood (CarnegieMuseum of Natural His-
but females are polychromatic (Williamson tory), and R. L. Zusi (U.S. National Museum). This
1956).Conversely,malesof promiscuous
species work was supportedin part by grantsfrom the De-
October1985]
FemaleVariationin Iridescence
partment of Biology, Harvard University, and by a
ChapmanMemorial Fund grant from the American
Museum of Natural History.
patterns.Pp. 319-364in Evolutionarybiology(M.
K. Hecht, W. C. Steere, and B. Wallaceß Eds.).
New York, Plenum Press.
EWALD,P. W., & F. L. CARPENTER.1978. Territorial
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strike me at first that it might be this bird; moreover,
its actionwassoodd that I hardly knew what to make
of it. At any rate one soonnoiselesslylit, like a great,
gray moth, directly in front of me in the road, but a
few feet distant.It wasextremelydifficultto seehim,
and it was more by good luck than good shooting
talli.--Just without the picket fence that enclosesin
part the parking of my presentresidenceat Fort Wingate,New Mexico,then [sic]runs a wide board-walk.
Beyondthis is a broad,well-kept gravel road,stand- that the little pinch of shot from my cane-gun
ing between the former and an open level plot of knockedhim over, though the weapon rarely fails
ground of about an acre's extent. For a number of me in daytime. I immediately ran up to my study
eveningspastmy neighborshave tried to induce me with my prize, where I discoveredI had killed a fine
to come out and see a strange-actingbird that dis- specimenof Nuttall's Poor-will. As the skeleton of
ported itself in this roadway,between twilight and this bird had long been amongmy desiderata,the
dark. I paid little heed to this, asfrom its description skin and its beautifulplumagewassoonstrippedoff,
I believedit to be the half-grownyoungof the Chor- whereuponI wasmuch surprisedto find in its mouth
dediles
of this region, which is very abundantin the somefour or five quite sizablemoths,and the upper
filled with a wad of a dozneighborhood.Lastnight, however,the bird having portionof the oesophagus
been described
to me as a small Owl
with
a white
throat, by one of its observers,I took my cane-gun
and made a searchfor it up and down the road-way.
I had not far to go, when, as well as I could seeby
the light of a very young moon, I noticed a small,
dark-brownish looking bird apparently amusing
himselfby making shortjumpsof two feet or more
up in the air, then restingon the road to repeatthe
performancein a momentor so.Another was going
through similar capers on the broad walk. They
seemedto be perfectlyobliviousto my presence,and,
indeed, some children further along were trying to
catch them with
their hands. As I had never
heard
the note of the Poor-willsin the vicinity, it did not
en or fifteenmore.Fully half of thesewere yet alive,
and two or three managedto fly away when freed
from the bodiesof their more disabledcompanions.
This, then, is what the bird was up to; instead of
flying about as a Nighthawk does, taking his insect
prey in a conspicuousmanner upon the wing, he
capturesit in the way I have describedabove.
"To-night the moon is twenty-four hours older,
andthe eveningproportionately
brighter,but a careful search for over half an hour failed to discover a
single specimenof the bird on the sameground. I
am not aware that any of the other Caprimulgidae
have similar habits.--DR. R. W. SHUFELDT,Fort Win-
gate,New Mexico."